This invention relates to rolls, and more specifically to a trimetal pulverizer roll of the type that is intended to be used in bowl mills for purposes of effecting the pulverization therein of material such as, for example, coal and to a method of manufacturing such a trimetal pulverizer roll.
An essential component of any steam generation system of the type, which utilizes pulverized coal as a fuel, is the apparatus in which the coal is pulverized in order to render it suitable for such usage. Although the prior art is replete with examples of various types of apparatus that have been employed for purposes of accomplishing coal pulverization, one form of apparatus, which in particular has frequently been used for this purpose, is that commonly referred to as a bowl mill. The latter obtains its name principally from the fact that the pulverization, i.e., grinding, of the coal that takes place therewithin occurs on a grinding surface, which insofar as configuration is concerned resembles that of a bowl.
For an exemplary showing of a prior art form of bowl mill, reference may be had to U.S. Pat. No. 3,465,971, which is assigned to the same assignee as the present invention. U.S. Pat. No. 3,465,971 contains a teaching of both the nature of the construction and the mode of operation of a bowl mill that is suitable for use for purposes of effecting the pulverization of the coal that is used to fuel a coal-fired steam generator. As set forth in this patent, the essential components of such a bowl mill are a body portion, i.e., housing, within which a grinding table is mounted for rotation, a plurality of grinding rolls that are supported in equally spaced relation one to another, the plurality of grinding rolls are suitably arranged in such a manner so as to coact with the grinding table such that coal which is disposed on the surface of the grinding table is capable of being ground, i.e., pulverized, by the rolls, coal supply means for feeding to the surface of the grinding table the coal which is to be pulverized in the bowl mill, and air supply means for providing to the interior of the body portion the air that is required in the operation of the bowl mill.
In order to satisfy the demands for pulverized coal of a coal-fired steam generation system that embodies a conventional form of construction, there commonly exists a need to employ a plurality of bowl mills of the type which are shown in the aforereferenced patent. In this regard, note is further made here of the fact that in terms of capacity each of these bowl mills may have a capacity of up to one hundred tons per hour of pulverized coal. In addition to possessing the capability of operating at its maximum capacity, each of these bowl mills must also be capable of operating at less than full capacity, i.e., at some percentage thereof, e.g., 25%, 50%, 75%, etc. Finally, these bowl mills must also be capable of use with a variety of types of coal that have dissimilar grinding characteristics.
With further reference to the aforementioned patent, the mode of construction of the grinding rolls that are depicted therein is such that each grinding roll is designed to be mounted on a shaft-like member. As a result the grinding rolls are each capable of movement relative to the surface of the grinding table. To effect the assembly of the grinding roll with the aforesaid shaft-like member, each grinding roll preferably has a through passage formed through the center thereof. The function of this through passage is to enable the shaft-like member to be positioned therewithin in mounted relation thereto. Accordingly, in order to facilitate the task of providing the grinding roll with such a through passage, it is desirable that the grinding roll be composed of a material that is characterized by its ease of machinability, i.e., a relatively soft material such as gray iron.
On the other hand, in addition to it being desirable to form the grinding roll of a material that is relatively soft, there exists also a need for at least the external, i.e., outer, surface of the grinding roll to consist of a material that is characterized by its good wear-resistant qualities. The primary reason for this is that the grinding rolls in the course of effecting the pulverization of material therewith are of necessity subjected to a harsh abrasive action, the latter being occasioned by virtue of the nature of the material that is being pulverized as well as by virtue of the manner in which this pulverization takes place. The result, therefore, is that the grinding rolls exhibit a susceptibility to being rendered unusable in a relatively short period of time. That is, the rolls are found to have a relatively short operating life. For obvious reasons, it is desirable that such a result be avoided, if possible. As is known to those who are skilled in this art, the wear, which the grinding rolls that are employed in bowl mills actually experience, is influenced principally by the grinding characteristics of the material that is being pulverized as well as by the productive output of the bowl mill, i.e., the amount of material that is being pulverized in the bowl mill in a given period of time.
When the external, i.e., outer, surface of the pulverizer, i.e., grinding, roll becomes sufficiently worn so as to preclude any further use thereof for purposes of effecting the pulverization of material therewith, the remaining portions of the roll are normally still functional. That is, the grinding roll but for its worn external surface would still be serviceable. Thus, from the standpoint of achieving economies of manufacture, it would be desirable, if possible, to effect a resurfacing of the external surface of the grinding roll such that the latter might once again be utilized for the pulverization of material rather than necessitating that the worn grinding roll be replaced by one that is entirely new. Further to this point, one would normally expect that the cost of reconditioning, i.e., resurfacing, the outer surface of a worn grinding roll would be significantly less than the cost associated with the manufacture of an entirely new grinding roll.
In addition to the relative cost of resurfacing the external, i.e., outer, surface of a worn grinding roll as contrasted to the cost of manufacturing an entirely new grinding roll, there also exists the matter of the operating life which is achievable with a resurfaced worn grinding roll as contrasted to that which is obtainable with an entirely new grinding roll. Namely, in order to be economically feasible it is desirable that the operating life of a resurfaced worn grinding roll be approximately equivalent to that which one would obtain from a new grinding roll. That is, any savings which are achievable from making use of a resurfaced worn grinding roll as contrasted to employing an entirely new grinding roll should not be dissipated as a consequence of the fact that the operating life of a worn grinding roll is such that several resurfacings thereof are required in order to realize the same operating life that can be realized with a grinding roll that is totally new. To summarize, if in the interest of obtaining comparable operating lives, multiple resurfacings would have to be had of the worn grinding roll such that the cost thereof would probably equal, if not exceed, the cost of providing an entirely new grinding roll, then, the advantages accruing from reusing a worn grinding roll vis-a-vis replacing the latter with a new grinding roll would undoubtedly not be realized.
It has been known in the prior art to resurface a worn grinding roll. Further, the results obtainable therefrom have proven to be generally satisfactory. Most often, this resurfacing has been in the form of providing the external surface of the worn grinding roll with a suitably dimensioned layer of weld metal. In view of this experience which the prior art has had with the resurfacing of a worn grinding roll, there have been attempts made at providing the external surface of new grinding rolls with a hard facing. Unfortunately, however, such attempts applied to new grinding rolls have not heretodate met with much success.
The motivation behind the attempt to effect a resurfacing of new grinding rolls has largely resided in the desire to extend the operating life thereof. One of the major reasons for seeking to extend the operating life of new grinding rolls is that it would enable the period to be extended between when shutdowns are required to accomplish the removal of worn grinding rolls and the replacement thereof with new grinding rolls. Furthermore, when consideration is given to the fact that each bowl mill normally embodies three grinding rolls, each of which periodically must be removed and replaced when it becomes worn, as well as the fact that a plurality of bowl mills are commonly employed to provide the requisite amount of pulverized coal to a coal-fired steam generation system, the magnitude of the problem that is presented by the need to effect a shut down of the bowl mill for purposes of accomplishing the removal and replacement of worn grinding rolls while concomitantly ensuring that the power generation system retains the capability to provide continuous uninterrupted service should be readily apparent.
In addition to the above there is also the matter of the expenditure of time and effort along with the cost per se that is associated with the act of removing and replacing a worn grinding roll. Obviously, therefore, if one were to be successful in achieving a reduction in the frequency with which grinding rolls are required to be replaced as a consequence of their having become worn, cost savings could be realized in terms of the time and effort that is required to be expended to effect such replacement. There has been shown to exist in the prior art a need for a new and improved form of grinding roll that is characterized by the fact that this new grinding roll possesses a significantly longer operating life than that commonly found to be possessed by prior art forms of grinding rolls. In addition, a need has been demonstrated for a new and improved method for manufacturing such a new and improved form of grinding roll.
It is, therefore, an object of the present invention to provide a new and improved form of pulverizer roll.
It is another object of the present invention to provide such a pulverizer roll which is characterized in that it embodies a trimetal mode of construction.
It is still another object of the present invention to provide such a trimetal pulverizer roll that is primarily intended to be employed in a bowl mill for purposes of effecting the pulverization therewithin of a material such as coal.
A further object of the present invention is to provide such a trimetal pulverizer roll that is distinguishable by its relatively long operating life.
A still further object of the present invention is to provide such a trimetal pulverizer roll having a first portion thereof that is characterized by its ease of machinability, a second portion thereof that is characterized by the fact that it exhibits medium wear-resistant qualities, and a third portion thereof that is characterized by the fact that it exhibits highly abrasive resistant qualities.
Yet another object of the present invention is to provide such a trimetal pulverizer roll that while being readily employable in a bowl mill yet also enables significant cost savings to be realized through the use thereof insofar as the operation of the bowl mill is concerned.
Yet still another object of the present invention is to provide a new and improved method of manufacturing such a trimetal pulverizer roll.